Get week number from a Date object

[Glimpse]

Hello all,

I'd like to know if there is an easy way to get the week number from a
Date object. In pure Java, the Calendar class does the trick via a
call to get(Calendar.WEEK_OF_YEAR); but the Calendar class is not
available in GWT.

Any idea?

[ctasada]

AFAIK the only way is calculating it yourself

public static int calendarWeekIso(final Date inputDate) {

int thursdayDay = 4 + firstDayOfWeek;

Date thisThursday = new Date(inputDate.getYear(),
inputDate.getMonth(),
inputDate.getDate() - weekday(inputDate) + thursdayDay);

Date firstThursdayOfYear = new Date(thisThursday.getYear(), 0, 1);

while (weekday(firstThursdayOfYear) != thursdayDay) {
firstThursdayOfYear.setDate(firstThursdayOfYear.getDate() + 1);
}

Date firstMondayOfYear = new Date(firstThursdayOfYear.getYear(), 0,
firstThursdayOfYear.getDate() - 3);

Long cw = (thisThursday.getTime() - firstMondayOfYear.getTime())
/ ONE_DAY / DAYS_IN_A_WEEK + 1;

return cw.intValue();

[Glimpse]

Thank you.

Maybe I made a mistake when coding it with your algorithm, but it
seems that it does not always work.
As a reference, I'm using this website (in french):
http://www.calendrier-365.fr/calendrier-2010.html

And I found this script to work well: http://syn.ac/tech/19/get-the-weeknumber-with-javascript/
It is easy to transform into Java/GWT (but with all the deprecation
warnings around the Date object).

[Stefano Ciccarelli]

If you need the ISO week this is what I'm using in GWT:

    private static final int ISO_THURSDAY = 4;

    private static final int MAX_DAY_OF_WEEK = 6;

    private static final int DAYS_IN_WEEK = 7;

    private static final long MILLIS_DAY = 86400000;

    @SuppressWarnings("deprecation")

    public int getWeek() {

        // http://en.wikipedia.org/wiki/Talk:ISO_week_date#Algorithms

        final Date date = toDate();

        // ISO week day (Mon=1 to Sun=7)

        final int dayOfWeek = 1 + (date.getDay() + MAX_DAY_OF_WEEK) % DAYS_IN_WEEK;

        final Date nearestThu = addDays(date, ISO_THURSDAY - dayOfWeek);

        final int year = nearestThu.getYear();

        final Date jan1 = new Date(year, 0, 1);

        return 1 + dayDiff(nearestThu, jan1) / DAYS_IN_WEEK;

    }

    public static Date addDays(final Date sourceDate, final long days) {

        return new Date(sourceDate.getTime() + (days * MILLIS_DAY));

    }

    public static int dayDiff(final Date firstDate, final Date secondDate) {

        return (int) ((firstDate.getTime() - secondDate.getTime()) / MILLIS_DAY);

    }

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